add arp support

[lieka1]

Hi there! i dont' know if you are accepting any pr for arp support

added: SlicedPacket::from_ethernet is now supporting arp EtherType PacketBuilderStep is added to build arp header

but LaxSlicedPacket::from_ethernet still cannot parse arp package, i am not sure if is should add it since arp header is quite short

[JulianSchmid]

Hi @lieka1 ,

I will have a look at your PR this weekend. Thanks in advance.

Greets Julian

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 79.33333% with 62 lines in your changes missing coverage. Please review.

Project coverage is 99.16%. Comparing base (2a23d8f) to head (67a1635).

Files	Patch %	Lines
etherparse/src/link/arp_payload.rs	65.34%	35 Missing :warning:
etherparse/src/link/arp_header.rs	82.00%	18 Missing :warning:
etherparse/src/packet_headers.rs	54.54%	5 Missing :warning:
etherparse/src/err/packet/build_write_error.rs	0.00%	2 Missing :warning:
etherparse/src/packet_builder.rs	98.82%	1 Missing :warning:
etherparse/src/payload_slice.rs	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #103 +/- ## ========================================== - Coverage 99.35% 99.16% -0.20% ========================================== Files 163 165 +2 Lines 29725 30019 +294 ========================================== + Hits 29534 29768 +234 - Misses 191 251 +60 ```

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